For liquid dispensing there are pipettes which comprise a piston by means of which liquid is aspirated into and removed from a pipette tip attached to an aspirating channel. The tip is sealed against the outer surface of the lower tip of the aspiration channel. The sealing is usually carried out so that the material of the tip is softer than that of the lower tip of the aspiration channel, whereby the tip yields and is thus sealed against the surface of the aspiration channel. The sealing may be carried out also, for example, by means of an O-ring in a surrounding groove on the outer surface of the aspiration channel. The width of the groove corresponds to the thickness of the O-ring. After dispensing, the tip is removed. For this, in the pipette there is usually a sleeve-like tip ejector, by means of which the tip is pushed off. In order to achieve reliable dispensing, the tip sealing must be sufficiently tight and proof so that the connection does not leak or the tip accidentally loosen. On the other hand, in order to achieve good usability and reliable operation of the tip remover, the forces needed for attaching and loosening should be as small as possible. Taking care of reliable sealing without increasing the release force too much is important especially in multichannel pipettes.
EP 2 452 754 describes a multichannel pipette in which the sealing has been carried out by means of an O-ring. The O-ring is placed in a groove the width of which in the axial direction corresponds to the thickness of the O-ring.